Perhaps not a forced dialog box, as any dialog box that pops up on opening a presentation is inhibitive to work flow. Given modern design, perhaps a message in the yellow information bar that can appear below the ribbon for messages (eg when an Office Update is available), with a button to then display a dialog box listing the fonts.

Better yet, this should be a FONTS REPLACEMENT dialog box - I think OpenOffice Impress has this feature?

This would make it much easier for people to understand why their presentation looks 'wrong'. Such a frequent issue these days with everyone wanting to differentiate themselves by using non standard fonts.